ARGUMENTS

All messages specified after publish will be published without retain flag.
I.e., they will be forwarded to clients subscribed to the topic, but they will
not be retained for clients subscribing at a later time.

This is the default behaviour.

retain

All messages specified after retain will be published and retained on the
MQTT broker as "last known good" value. So, clients which subscribe at a later
time will still get the value published this way -- unless it is updated or
deleted in the meantime.

topic=value

Publish (and optionally retain, see above) value as a message for topic.

If value begins with a / (slash), it is interpreted as a filename
and the file content published instead. If the file contains a trailing
newline character, it will be left out of the message body.

To avoid value being parsed as a filename, prefix it with an additional
slash character: Use //... instead of /.... This additional
character will not be part of the published message.